Reheat gas turbine combined with steam turbine

ABSTRACT

A process and apparatus for generating useful power comprises the use of a combined reheat gas turbine and steam turbine cycle. The combined cycle optionally includes the superheating of steam and the reheating of steam in the reheat combustor of the reheat gas turbine. The use of second generation high-ratio, high-firing temperature gas generators in the combined cycle of the present invention yields increased efficiency and output heretofore unexpected from reheat gas and combined cycles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and process for generating usefulpower. More particularly, the invention provides a combined reheat gasturbine and steam turbine cycle wherein a reheat combustor accepts aheated and compressed gas produced by a conventional gas generator, addsfuel and delivers reheated gas to a power turbine for directlygenerating power, exhaust gas from the power turbine forming superheatedsteam to drive the steam turbine. Alternatively, the reheat combustorfor reheating the gas generator exhaust gas incorporates heat exchangemeans for superheating steam therein prior to delivery of reheated gasto the power turbine and optionally for production of reheated turbinesteam for return to the steam turbine to furnish additional powerthereto.

2. Description of the Prior Art

Research and development is currently being directed toward manyconfigurations of power systems involving gas turbines because of thegrowing awareness of impending world energy shortages. The presentinvention relates to the need for focus technical attention to thereheat cycle and with use of the apparatus and processes of the presentinvention, the reheat gas turbine cycle and combined gas reheat andsteam reheat cycle can appreciably increase power plant thermalefficiency to approximately an over-all 50% efficiency level or higher.

The reheat gas turbine cycle itself is well-known and has receivedconsiderable attention over the years, particularly in Europe. Aregenerator has been used to heat a compressor's discharge air toimprove cycle efficiency, and intercooling has been suggested for thesame purpose. It is known that reheating can increase power output by 35to 40%, but without use of regeneration, over-all cycle efficiency isdegraded.

Another example of a reheat gas turbine cycle presently employed is theafterburner of a jet engine for aircraft use, such as in militaryaircraft and in certain commercial supersonic planes. The jet reheatcycle has been developed to get reliable service in applications whereaugmented power output is required for a short or limited time. Thegreatly increased power output comes at the expense, however, ofmarkedly increased fuel consumption, owing to which, commercialapplications have gone to efficient high bypass ratio fan jets forsubsonic flight. Technology developed therefrom has made availablesecond generation high-ratio high-firing temperature gas generators forindustrial applications.

Yet, despite the existence of known technology, combined cycle powerplants, such as those which utilize the processes and apparatus of thepresent invention, have not been developed. Attention has not been givento reheat gas turbine combined steam turbine cycle power plants becauseit has been thought that the degradation in efficiency of the reheatcycle would not offer an advantage, and also that increased fuelconsumption would result. However, as will be pointed out hereinafter,thought must be given to the existence of higher level heat available inthe exhaust and the over-all entropy changes as well as the concept ofextracting maximum work at the high working fluid temperature levelspossible with the present invention to obtain highest efficiency.Another explanation of the failure of others to utilize a reheat cycleprocess is the feeling that greater complication and cost of controls,and additional burner, compatibility of nozzle area, start-upprocedures, and the like, would offset any advantage in specific poweroutput, particularly at a higher fuel consumption.

Yet another reason can be cited for the failure of others to point inthe direction of present invention. Aircraft derived gas turbines offeran advantage in potential physical arrangement for the reheat cycle inthat a reheat combustor can be readily added between the gas generatorand the power turbine, whereas conventional industrial or heavy duty gasturbines are ordinarily single shaft units for power generation, suchconventional units not readily lending themselves to addition of areheat burner. Moreover, second generation aircraft gas turbines fire atelevated temperatures and utilize high compression ratios suitable forreheat cycles, while industrial units, because of a single shaftconfiguration, are limited to lower ratios.

With the appearance of an increasing number of high-temperature andhigh-pressure-ratio gas turbines which lend themselves to reheat cycles,use of such equipment according to the process and teachings of thepresent invention becomes technically feasible, affording a practicalutility for the generation of useful power, such as electrical power.

SUMMARY OF THE INVENTION

The invention contemplates a process and apparatus for generating usefulpower by utilizing a combined reheat gas turbine and steam turbinecycle. The process comprises generating a compressed, heated gas in agas generator arrangement, and then reheating the exhaust gas forgeneration of power in a second turbine, followed by extraction by heatexchange of the useful energy in the exhaust gas from the second turbineto indirectly power a steam turbine through generation of steam.

Accordingly, it is an object of the invention to provide a process forreheating gas generator exhaust gas to generate power in a powerturbine, followed by extraction of energy from the power turbine exhaustto generate steam for powering a steam turbine.

A further object of the invention is to provide a reheat gas turbineincluding a reheat annular combustor with which an associated gasgenerator is easily installable and easily removable for servicing.

Still another object of the invention is to provide a reheat gas turbineincluding a reheat combustor with a better and longer diffuser having atwo-shaft construction, the reheat combustor being associated with a gasgenerator in such a manner so as to produce an axial flow of gasthroughout for lower pressure loss, which results in higher over-alloperational efficiency.

Also contemplated within the scope of the invention is a combined reheatgas turbine and steam turbine cycle for production of useful powerwherein; superheated steam is produced by heat exchange in the reheatcombustor and reheated gas passing therethrough drives a power turbinefor direct production of useful power. The superheated steam producesuseful power by driving a steam turbine. Optionally, the combined cyclecan include provision of additional heat exchange means in the combustorcavity for reheating exhaust steam produced by the steam turbine andreintroducing the reheated steam into the steam turbine.

An important object of heating the steam in the combustor cavity is toshift the heat load from the heat recovery boiler to the reheatcombustor thus allowing a minimum heat recovery boiler stacktemperature; that is a minimum stack loss and a higher cycle efficiency.This shift in heat load also affords greater temperature differentialsbetween the exhaust gasses and the boiling water in the evaporator andthe preheated water in the economizer to reduce back pressure and toreduce the heat exchange tube surface required and thus lower boilersize and cost.

Accordingly, yet another object of the invention is to provide acombined reheat gas turbine combustor cavity which includes a steam heatexchanger, the combustor cavity delivering reheated gas to a powerturbine and simultaneously transferring heat by heat exchange to a steamsystem for powering a steam turbine.

A further object of the invention is to operate a combined reheat gasturbine and steam turbine cycle at pressure ratios and temperaturesunique for such combined cycle providing a high cycle efficiency.

Still a further object of the invention is to provide optimum control ofnitrogen oxide emissions with two burners having different ratios of airand fuel, or by injection of water or steam into the reheat burner.

Another further object of the invention is to provide a process ofoperating a reheat combustor cavity in which steam superheater andreheater coils are placed.

These together with other objects and advantages which will becomesubsequently apparent reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a gas generator with a tandem reheat gaspower turbine driving an electric generator, along with a steam turbinedriving a second electric generator in a combined reheat gas powerturbine and steam turbine cycle.

FIG. 2 is a side elevational view, partly in diagrammatic section, ofthe axial flow reheat gas turbine of the present invention illustratinga gas generator with an associated second reheat combustor.

FIG. 3 is a schematic view of a combined reheat gas turbine and steamturbine cycle in which the reheat combustor cavity associated with thereheat gas power turbine and provides for steam superheating and steamreheating. A tandem power turbine arrangement is shown.

FIG. 4 is a longitudinal sectional view of an axial flow reheatcombustor cavity for effecting gas reheating, steam superheating andsteam reheating.

FIG. 5 is a transverse sectional view of the combustor of FIG. 4 showingin addition details of the mounting support and gib key.

FIG. 6 is an enlarged transverse sectional view of a tube forsuperheating steam and/or reheating in the combustor cavity of FIG. 4.

FIG. 7 is a diagrammatic view of the steam path heat balance in aconventional reheat steam turbine power plant cycle.

FIG. 8 is a diagrammatic view of the heat balance in a reheat bottomingcycle for the same steam conditions used in calculations presentedherein.

FIG. 9 is a graph showing gas turbine cycle efficiency as a function ofnet power output for various cycle pressure ratios and firingtemperatures for each of three conditions of no reheat, partial reheatand full reheat.

FIG. 10 is a graph showing power turbine expansion ratios as a functionof cycle pressure ratios for various firing temperatures of a gasgenerator combined with a power turbine.

FIG. 11 is a graph showing gas generator and power turbine exittemperatures as a function of cycle pressure ratios for different firingtemperatures of a gas generator and associated power turbine.

FIG. 12 is a graph showing the fuel ratio of the first combustor and thereheat combustor as a function of cycle pressure ratio for variousfiring temperatures.

FIG. 13 is a graph showing combined reheat gas turbine and steam turbinecycle efficiency as a function of firing temperature for a full reheatcycle and for a simple cycle.

FIG. 14 is a graph showing the combined output as a function of firingtemperature for both a simple cycle and a full reheat cycle gas turbine.

FIG. 15 is a graph of combined cycle efficiency showing the percentageconversion of heat to steam under the optimum pressure ratio for simplecycle efficiency and output, for three firing temperatures in theleft-hand portion of FIG. 15, and combined cycle efficiency for optimumpressure ratios for reheat gas cycle efficiency and output as a functionof reheat temperature rise for three firing temperatures in theright-hand portion of FIG. 15.

FIG. 16 is a graph showing the ratio of gas turbine output to steamturbine output as a function of reheat temperature rise for the optimumpressure ratio for efficiency and the optimum pressure ratio for output,for various firing temperatures and for full and partial reheat.

FIG. 17 is a graph showing the relationship of temperature and enthalpyof both the steam being generated and the gas turbine exhaust gases.This graph also presents the effect the boiler pinch point has on finalstack temperature.

FIG. 18 is a graph showing the temperature differentials between the gasgenerator bypass gas around the combustor cavity and the steam beingsuperheated. It also shows the temperature differentials between thecombustion cavity gas and the steam being superheated.

FIG. 19 is a fragmentary sectional view of a modified form of thecombustion cavity which can be utilized in the combined cycle of FIG. 3,showing its adaptation for burning of finely divided solid fuel, such aspowdered coal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a schematic diagram of the combined cycle of the presentinvention shows gas generator 20 which receives air through inlet line22, producing compressed air by compressor 24, which is driven throughshaft 26 by gas generator turbine 28, which is powered by gas producedin first combustor 30 from air entering combustor 30 through line 32 andfuel entering combustor 30 through fuel line 34. Reheat or secondcombustor 36 receives exhaust from gas generator turbine 28 throughreheat inlet line 38 and discharges reheated gas through line 40 topower turbine 42, which drives first electric generator 44 directly byshaft 46. A second identical power turbine 42' can be used in tandemwith power turbine 42 to drive electrical generator 44 by shaft 46'.Reheat gas output leaves power turbine 42 through exit line 48 andpasses into heat exchanger 50 prior to discharge through stack line 52.Exit gases through exit line 48 pass through three stages of heatexchanger 50, the first being superheater 54, where superheated steam isproduced through superheat line 56; the second being evaporator 58,where water from storage drum 60 and recycled water is evaporated inline 62; and the third being economizer 64. In economizer 64, recycledwater entering from line 66 is warmed for evaporation in line 59 beforeentering evaporator 58. Superheated steam leaving superheater 54 throughline 56 enters and drives steam turbine 68, which directly powers secondelectric generator 70 through shaft 72. Condensate from steam turbine 68is formed and collected in condenser 76 and pumped by condenser pump 78through line 80, along with steam formed directly in turbine 68, throughline 82, to heater 86, the output of which is fed by boiler feed pump 88to recycle line 66. It is to be particularly noted that a tandemarrangement of gas generator and turbine combination can be utilized,with first electric generator 44 being powered by two such arrangements,the combined reheat exhaust gas output of which feeds into exit line 48.It should further be noted that each of the individual components of thesystem shown in FIG. 1 with the exception of the reheat combustor isconventional and only the combined arrangement including the reheatcombustor leading to the advantages and efficiencies disclosed in thepresent invention, are intended to be described as new.

In the schematic diagram of FIG. 3, gas generator 20 functions in thesame manner as the gas generator described for FIG. 1, and steam turbine68, condenser 76, heater 86, and pumps 78 and 88 function in the samemanner as in FIG. 1. However, the output of gas generator 20 enterscavity 90 through line 38, cavity 90 being, a combined reheat combustorand superheater, reheating exhaust gas from line 38 of gas generator 20and discharging reheated gas through line 40 to drive power turbine 42,which drives first electric generator 44 by means of shaft 46. Cavity 90also superheats the output from evaporator 58, entering cavity 90through line 92 and leaving through line 56, for driving steam turbine68. Reheating of steam also occurs in cavity 90, the steam enteringcavity 90 through line 94 and leaving cavity 90 through line 96 to entersteam turbine 68. Steam turbine 68 drives second electric generator 70through shaft 72, and recycling of condensed output through line 66occurs through economizer 64 and evaporator 58. A tandem power turbinearrangement equivalent to that shown in FIG. 1 can be used in thecombined cycle of FIG. 3.

FIG. 2 is a representation of the reheat gas turbine cycle of thepresent invention including gas generator 20 in association with powerturbine 42. Gas generator 20 is made up of combustor 30 and aircompressor 24, which has stages 100, 102 and 104, 106 and 108 of afive-stage low pressure section, as well as stages 110 and 112, whichare representative stages of a 14-stage high pressure section. Combustor30 discharges compressed heated air to stages 114 and 116 of a two-stagehigh pressure section and stage 118 of a one-stage low pressure section.Shaft 26 connects compressor 24 with gas generator turbine 28. A numberof high temperature and high pressure ratio gas turbines are now on themarket, gas generator 20 in FIG. 2 illustrating the second generation LM5000 Model, other model designations currently available commerciallyincluding the LM 2500, JT 9, RB-211, Spey, and the Mars.

Gas generator 20 in FIG. 2 is coupled with reheat combustor 36, showingfuel line 35, fuel nozzle 120, annular combustion region 122, diffuser124, and the combustion region of power turbine 42. It is to beunderstood that a plurality of fuel nozzles 120, arranged concentricallyproduce the annular flow of reheat gas which drives power turbine 42.

Alternatively, the output of gas generator 20 can feed cavity 90 shownin FIGS. 4 and 5, for performing gas reheat and both steam reheat andsuperheating functions. Gas from diffuser 130 passes around struts 132and is heated in combustion region 134 as fuel nozzle 136 dischargesfuel for combustion therein. Fuel enters cavity 90 through fuel line 138and is ignited by spark plug 140. Superheat and steam reheat helicalcoils 142 are shown somewhat schematically in FIG. 4, having theconfiguration of FIG. 6 in enlarged detail. Boiler steam header 144furnishes steam through line 92 to cavity 90, and after boiler steam hastraversed its helical path through cavity 90, it leaves cavity 90through line 56 for collection in boiler steam header 148. Reheat steamleaves cavity 90 through reheat steam outlet line 96, entering reheatsteam header 146. Boiler steam enters cavity 90 through line 92 andheader 144, while boiler steam exits cavity 90 through line 56 andheader 148. Reheat steam enters cavity 90 through reheat steam inletline 94 from header 150. Insulation 152 surrounds cavity 90, diffuser130, headers 146, 148, 144 and 150, and also surrounds inlet powerturbine nozzle 154. Exit gases pass through inlet nozzle 154 to driveturbine 42, passing next to power turbine first stage nozzle 156 asshown in FIG. 4. The inside surface 157 of inlet nozzle 154 is in theshape of a nose of a bullet, while outside bell mouth 196 has the shapeshown. FIG. 6 shows streamlined fabricated superheat tube 142 containingperforated sheath 158 surrounding three tubes 160, 162 and 164 forcontaining superheated and/or reheated steam, the respective diametersvarying according to the flow rates and throughput demanded of eachcomponent.

The advantages of the cycle of the present invention, such as isaccomplished with the combined superheating and reheating apparatus ofFIGS. 4, 5 and 6, is illustrated by the established and industryaccepted data of the steam path heat balance of a conventional reheatsteam turbine with steam conditions of 2400 psig and 1000/1000° F. forreference shown in FIGS. 7 and 8. The steam path heat balance in FIG. 8can be used in accordance with the combined cycle of the presentinvention and, compared with the heat balance of a conventional reheatsteam turbine in FIG. 7. Certain assumptions have been made indeveloping the data of FIGS. 7 and 8, for the sake of concreteness. Bothsteam turbine heat balances take into account the boiler feed pumpinput, but a loss of 3% should be applied to each to take intoconsideration steam leakage, mechanical losses and the hydrogen-cooledelectric generator losses. By calculating the net output, FIG. 7 shows497 BTU's, and by determining the net input from the indicated data, anet input of 1165 BTU's is seen to result, giving a net efficiency of42.7% and applying an established boiler efficiency of 89% along withthe 3% losses given above results in a conventional power plant cycleefficiency of 36.86%. Similarly, from FIG. 8, a gross output of 581BTU's is obtained from a net input of 1437 BTU's, giving a netefficiency of 40.43% and applying the 3% losses given above results in acycle efficiency of 39.22% where no boiler efficiency is applicable.Utilizing these data, and making the 16 assumptions listed in Table I,the cycle efficiency of the combined cycle with additional steam reheatcan be calculated as shown in Table II.

                                      TABLE I                                     __________________________________________________________________________    Assumptions in Efficiency Calculations-                                       Gas Generator and Steam Reheat                                                No.                                                                              Item               Value                                                                              Units                                              __________________________________________________________________________    1. Inlet Pressure     14.7 psia                                               2. Ambient Temperature                                                                              60   degrees F.                                         3. Power Turbine Efficiency                                                                         87   percent                                            4. Reheat Gas Turbine Combustion Loss                                                               3    percent                                            5. Steam Superheater Combustion Loss                                                                1    percent                                            6. Steam Reheat Combustion Loss                                                                     1    percent                                            7. Reheat Combustor Pressure Drop                                                                   3    percent                                            8. Power Turbine & Generator Loss                                                                   2    percent                                            9. Steam Turbine & Generator Loss                                                                   3    percent                                            10.                                                                              Boiler Radiation & Blowdown Loss                                                                 2    percent                                               Constant Reheat Firing Temperature                                                               1800 degrees F.                                            Inlet & Back Pressure Loss                                                                       4/6  inches of water                                       Condenser Pressure 2    inches Hg absolute                                    Liquid Fuel with 18,400 BTU/lb. LHV                                           Keenan and Kaye Air Tables for                                                200% Theoretical Air                                                          Steam Heat Balance, FIG. 8.                                                __________________________________________________________________________

The cycle analysis can be carried out for three modes of operation,namely, continuous, electric base and peak operation, giving the resultsshown in Table II for half plant capacity. In this arrangement, twomodel LM 5000 gas generators are mounted to a single hydrogen-cooledelectric generator of 100,000 KW capacity, the power turbines havingoppositely rotating power shafts. The steam turbine is also a 100,000 KWcapacity, with the steam output for the two boilers and the two gasturbine superheaters feeding the one steam turbine:

                                      TABLE II                                    __________________________________________________________________________    Steam Reheat Cycle Efficiency                                                        Gas Gene-                                                                              Reheat Fuel                                                                          Gas Tur-                                                                           Steam Total                                              rator Fuel                                                                             Input LHV                                                                            bine Net                                                                           Turbine                                                                             Net Net                                            Input LHV                                                                              BTU/HR Output                                                                             Net   Output                                                                            Cycle                                   Mode   BTU/HR × 10.sup.6                                                                × 10.sup.6                                                                     KW   Output KW                                                                           KW  Eff %                                   __________________________________________________________________________    Continuous                                                                           296.24   337.75 43,608                                                                             48,234                                                                              91,842                                                                            49.44                                   Electric Base                                                                        320.16   334.14 46,601                                                                             49,313                                                                              95,914                                                                            50.03                                   Peak   336.72   328.22 48,750                                                                             49,561                                                                              98,311                                                                            50.46                                   __________________________________________________________________________

Results of Table II are to be compared with a similar arrangement inwhich steam reheat is absent but a reheat gas turbine, such as is shownin the schematic arrangement of FIG. 1, is utilized in which two modelLM 5000 gas generators are mounted in the same manner as above. TableIII presents the assumptions applied in the calculations of the combinedcycle efficiency:

                                      TABLE III                                   __________________________________________________________________________    Assumptions in Efficiency Calculations                                        Gas Generator Reheat Only.                                                    No.                                                                              Item              Value                                                                              Units                                               __________________________________________________________________________    1. Ambient Temperature                                                                             60   degrees F.                                          2. Inlet Pressure    14.7 psia                                                3. Steam Pressure    1250 psig                                                4. Steam Temperature 900  degrees F.                                          5. Feed Water Temperature to Boiler                                                                250  degrees F.                                          6. Steam Turbine Efficiency                                                                        80   percent                                             7. Power Turbine Efficiency                                                                        87   percent                                             8. Reheat Gas Turbine Combustor                                                  Loss              3    percent                                             9. Reheat Combustor Pressure Drop                                                                  3    percent                                             10.                                                                              Power Turbine & Generator Loss                                                                  2    percent                                                Steam Turbine & Generator Loss                                                                  3    percent                                                Boiler Radiation & Blowdown Loss                                                                2    percent                                                Constant Reheat Firing Tempera-                                               ture              1800 degrees F.                                             Inlet & Back Pressure Loss                                                                      4/10 inches of water                                        Condenser Pressure                                                                              2    inches Hg absolute                                     Keenan & Kaye Air Tables for                                                  400% Theoretical Air                                                       __________________________________________________________________________

and Table IV shows the results of the calculations of cycle efficiencyunder three modes of operation for half plant capacity.

                                      TABLE IV                                    __________________________________________________________________________    Gas Turbine Reheat Cycle Efficiency.                                          Half Plant Capacity                                                                  Gas Gene-                                                                              Reheat Fuel                                                                          Gas Tur-                                                                           Steam Total                                              rator Fuel                                                                             Input LHV                                                                            bine Net                                                                           Turbine                                                                             Net Net                                            Input LHV                                                                              BTU/HR Output                                                                             Net   Output                                                                            Cycle                                   Mode   BTU/HR × 10.sup.6                                                                × 10.sup.6                                                                     KW   Output KW                                                                           KW  Eff %                                   __________________________________________________________________________    Continuous                                                                           296.24   171.57 43,002                                                                             23,442                                                                              66,444                                                                            48.69                                   Electric Base                                                                        320.16   162.57 45,969                                                                             23,814                                                                              69,783                                                                            49.34                                   Peak   336.72   155.90 48,083                                                                             23,734                                                                              71,817                                                                            49.76                                   __________________________________________________________________________

The three modes of operations for which the calculations above arepresented are defined by the new International Standard ISO/DIS-3977 forgas turbines, and are as follows. The continuous mode of operationrepresents continuous or base load operation typical of pipelineservice, process applications or electric power generation up to 8700hours per year with infrequent starting greater than 100 hours perstart. The electric base mode of operation refers to longer duration ofmid-range electrical power generation, with process applications up to4000 hours per year and with up to eight hours per start. The peak modeof operation refers to short duration intermittent type of operation asoccurring typically in electrical power generation for peak loaddemands, where operation should normally be limited to 500 hours peryear and up to two hours per start.

The assumptions in the calculations above and used in the cycle arebased on figures for the Model LM 5000 gas generator obtained from themanufacturer after the first second generation unit was thoroughlytested. Characteristics of the LM 5000 gas generator are shown in TableV for base load ratings of 60° F. and 14.7 psia inlet pressure:

                  TABLE V                                                         ______________________________________                                        Characteristics of Gas Generator.                                             Base Load Ratings - 60° F. and 14.7 Psia Inlet                                               LM5000                                                  ______________________________________                                        Cycle Pressure Ratio P.sub.2 /P.sub.1                                                                 29                                                    Exhaust Flow #/sec.     272                                                   IGHP × 10.sup.3 * 50.00                                                 Cycle Eff. (IGHP)* LHV %                                                                              43.0                                                  Firing Temp. F.         2,100                                                 Gas Generator Ex Pressure Psia                                                                        56.10                                                 Gas Generator Ex Temp. F.                                                                             1,215                                                 ______________________________________                                         *Isentropic Gas Horsepower for 100% Expansion Efficiency.                

It is clear from comparison of the results of Table II and Table IV thatsteam reheat generates an over-all power plant efficiency approaching orexceeding 50% LHV when burning the distillate fuel in the apparatus ofFIG. 4. Even without incorporation of the superheater in the secondcombustor, it is possible with an arrangement such as that of FIG. 2 ora process shown in FIG. 1 to obtain net cycle efficiencies approachingor exceeding 49%. These efficiencies represent improvements over theefficiencies obtainable with use of conventional electric powergenerating equipment, and can lead to considerable cost savings inactual operation. A simplified economic evaluation for half plantcapacity and one power turbine making certain reasonable assumptions asto unit size, fuel cost, and the results of earlier calculations, theeconomic data of Table VI provide a measure of the degree of commercialsuccess expectable with both the reheat gas turbine and with the reheatgas turbine combined with the reheat steam turbine:

                                      TABLE VI                                    __________________________________________________________________________    Comparative Economic Analysis of Units Utilizing Reheat Cycles.                     Cycle                                                                             %        Fuel Cost*                                                                          Savings*                                                                           Savings*                                                                          Savings*                                          Eff.                                                                              Inc.                                                                             % Fuel                                                                              $/yr. ×                                                                       $/yr.                                                                              $ × 10.sup.6 /                                                              $ × 10.sup.6 /                        Unit  LHV Eff.                                                                             Consump.                                                                            10    10.sup.6                                                                           5 yrs                                                                             5 yrs.                                      __________________________________________________________________________    Conven-                                                                       tional                                                                        Combined                                                                      Cycle 44  Base                                                                             100   12.41                                                                              Base  Base                                                                              --                                          Reheat                                                                        G.Turb./                                                                      S.Turb.                                                                       (FIG. 2)                                                                            48.5                                                                              10.2                                                                             90.7  11.26                                                                              1.15  5.75                                                                              Base                                        Reheat                                                                        G.Turb/                                                                       Reheat                                                                        S.Turb.                                                                       (FIG. 4)                                                                            49.5                                                                              12.5                                                                             88.9  11.03                                                                              1.38  6.90                                                                              1.15                                        __________________________________________________________________________     *Based on Fuel Cost of $2.50 × 10.sup.6 BTU LHV.                   

Table VI indicates that there would be generation of 5.75 milliondollars in five years for amortization of the capital cost of the reheatpower turbine, while an additional 1.15 million dollars would be madeavailable in five years of operation to amortize the capital cost of areheat gas turbine combined with a reheat steam turbine system. Itshould further be noted that a savings in the heat recovery boiler wouldresult, since the boiler would be simpler and have less surface andgreater mean temperature difference. Moreover, the two superheaterswould not be duplicated in the heat recovery boiler, effecting evenfurther savings. These figures which are based on the standard industryaccepted combined cycle available today having a cycle efficiency of 44%LHV, provide a clear indication of the degree of commercial successexpectable from the two embodiments of the present invention, but theexistence of uncertainties, such as rising fuel costs, limited futureoil supply, coal fuel development, high interest rates, power rateincreases, and other uncertainties, could result in a shift favoring theembodiments of the present invention to an even greater extent in thefuture.

Considerations incident to the reheat turbine combustor of the presentinvention are the following. Combustor 36 incorporates an annular liner121 flaring outwardly to an exit plane 170 at power turbine 42. Fuelnozzles 120 are disposed in liner 121 and have orifices 123 equallyspaced on an orifice plane and separated by a chord width having a chordwidth diameter with respect to axis 172 of the entire arrangement. Fuelnozzles 120 are directed outwardly along a pitch line forming a pitchangle with respect to axis 172, the intersection of the pitch lines withexit plane 170 having a pitch line diameter in a certain preferred ratioto the chord diameter. This ratio is determined by calculating thevolume ratio of gases at the exit plane and at the orifice plane byapplying Boyle's law, PV=WRT. Under typical conditions, the output gasat the exit plane of the gas generator combustor 30 has a temperature ofapproximately 2100° F. (1149° C.) and a pressure of approximately 30atmospheres, while the exhaust gas at the orifice plane of the reheatcombustor 36 has a temperature of approximately 1800° F. (982° C.) and3.75 atmospheres. Accordingly, a volume ratio of 6.9 is obtained, andthis value is preferably increased slightly to produce a lower pressuredrop combustor 36.

Moreover, the length of diffuser 124 and combustor 36 should be about60% greater than the diffuser of a conventional power turbine notincorporating combustor 36. Preferably, diffuser 124 and in thealternative apparatus, diffuser 130, has a length of at least about fourfeet to prevent separation and resulting turbulence of incoming gas. Ofcourse, diffuser 124 or 130 will be constructed of proper angular changeand cross-sectional area change to accomplish the above. The powerturbine diameter can be slightly greater, and the nozzle chord widthadjusted to give approximately 1.35 times the first stage power turbinenozzle area.

Referring again to FIGS. 4 to 6, the required diameter of cavity 90 isabout 10 feet, which takes into account a gas velocity of about 100 feetper second and the increase in area required to accommodate superheatertubes 142, assuming an average gas temperature of about 2000° F. (1093°C.). The increase in area to accommodate tubes 142 is about 70%. Thelength of cavity 90 can vary somewhat according to the surface arearequired for superheating, and a workable length is about 18 feet. Outershell 176 is fabricated in sections about two feet long and boltedtogether at joints 178 so that superheater coils 142 can be insertedproperly. The outer shell 176 and associated pipes 56, 92, 94, 96, etc.,can be disconnected from headers 144, 146, 148 and 150 at flanges 57,93, 95, 97, etc., for removal of the two foot sections of outer shell176. Superheater tubes 142 are wrapped around in a manner to formhelical coils of specific configurations, such as that illustrated inFIG. 5. Coils 142 form an annular combustion cavity and control air flowto burner 136. Downstream from burner 136 coils 142 then temper hotcombustion gases and distribute them through more coils 142. Combustioncap 155 can also be seen.

In the arrangement shown in FIG. 5, cold steam enters at the bottom ofcavity 90, circulates through tubes 142 and then exits at the top foreach superheater. Shell 176 can be center-line supported by supportingmounts 180 to control expansion and to maintain a reasonable center-lineelevation with respect to foundation 182 and to satisfy axial alignmentof gas generator 20 and power turbine 42. Furthermore, gas generator 20can be quickly and readily removed to service cavity 90.

Superheater coils 142 are individually configured so that each tubelength is balanced for steam pressure drop and for equal radiant andconvection heat absorption area. Equal pressure drop is needed for flowdistribution and the balance or radiation and convection heat absorptionprovides good superheater temperature control, particularly at partialload conditions. The gas side pressure drop of conventional superheatersis about two inches of water per superheater, amounting to less than aone percent drop for the two superheaters in an atmospheric aspiratedboiler. This percent pressure loss can be applied to superheater coils142 operating at approximately 3.75 atmospheres. Tubes 142 arefabricated with three different sizes 160, 162 and 164, and arestreamlined with perforated leading and trailing edge overlays 158 tominimize pressure drop to control the hot gas flow as illustrated inFIG. 6. The perforations allow proper temperature gas to enter and reachthe interior surfaces of tubes 160, 162 and 164, thereby increasingconsiderably the effective heat transfer area. Diffuser 130 from gasgenerator 20 to cavity 90 has an extra length, and the gas generatorassisted by the angle of the struts 132 imparts a swirl to gas passingtherethrough to give the gas a longer flow path to assist in diffusionof the gas entering cavity 90.

Power turbine 42 has a nozzle consisting of an outer cone or bell mouth196 and inner bullet-shaped nose 157 to reduce inlet loss from cavity 90to power turbine 42, where velocity of gas is increased.

Data are shown in Table VII comparing a non-heat steam turbine cycle anda reheat steam turbine cycle each combined with the reheat gas turbinecycle, where pertinent data on the gas generator power turbine andboiler are included. As in previous comparisons, calculations have beenmade for three modes of operation, namely, continuous, electric base andpeak. It is seen that under all modes of operation, the cycle efficiencyis improved by use of steam reheat, and total net output is increasedsubstantially.

                                      TABLE VII                                   __________________________________________________________________________    Half Plant Capacity                                                           Non-Reheat Steam Turbine Steam Conditions 1250 pisg - 900° F.          Reheat Steam Turbine Steam Conditions 2400 psig - 1000/1000° F.        Average Gas Generator Figures Presented                                                            Continuous Electric Base                                                                            Peak                                                    No STRH                                                                             STRH                                                                              No STRH                                                                              STRH No STRH                                                                             STRH                         __________________________________________________________________________    1. GG IHP × 10.sup.3                                                                         50.0       55.0       59.1                               2. GG Firing Temperature °F.                                                                2100.sup.A 2250.sup.A 2400.sup.A                         3. GG Exhaust Flow lb./Sec.                                                                        272        283        289                                4. GG Exhaust Temperature °F.                                                               1215       1262       1295                               5. GG Exhaust Pressure psia                                                                        56.10      58.60      61.0                               6. GG Fuel Cons.BTU/HR × 10.sup.6 LHV                                                        296.24     320.16     366.72                             7. Fuel Cons.BTU/HR × 10.sup.6 LHV                                                           169.55                                                                              337.75                                                                             162.57                                                                              334.14                                                                             155.90                                                                              328.22                       8. Tot.Fuel Cons.BTU/HR × 10.sup.6                                         LHV               465.79                                                                              633.99                                                                             482.73                                                                              654.30                                                                             492.62                                                                              664.94                       9. PT Ex.Flow to Boiler lb./Sec.                                                                   274.5 277.2                                                                              285.4 288.1                                                                              291.3 294.0                        10.                                                                              PT Ex. Temp. to Boiler °F.                                                               1269  1281 1253  1266 1240  1252                            PT Expansion Ratio                                                                              3.702      3.867      4.025                                 PT Net Output BTU/LB. Flow                                                                      151.3 152.2                                                                              155.8 156.5                                                                              159.7 160.4                           Steam Flow lb./HR ×10.sup.3                                                               197.5 292.1                                                                              200.5 298.6                                                                              199.9 300.1                           Boiler Stack Temp. °F.                                                                   328   300  335   300  341   300                             Air/Fuel Ratio lb. Air/#Fuel                                                                    38.40 28.21                                                                              38.42 28.34                                                                              38.32 28.39                           PT Net MW Output  43.00 43.61                                                                              45.97 46.60                                                                              48.08 48.75                           ST Net MW Output  23.44 48.23                                                                              23.81 49.31                                                                              23.73 49.56                           Total Net MW Output                                                                             66.44 91.84                                                                              69.78 95.91                                                                              71.81 98.31                           Cycle Efficiency LHV %                                                                          48.69 49.44                                                                              49.34 50.03                                                                              49.76 50.46                        __________________________________________________________________________     *Estimated Values.                                                            Notations:                                                                    STRH  Steam turbine reheat                                                    GG  Gas generator                                                             IHP  Isentropic horsepower                                                    PT  Power Turbine                                                             ST  Steam turbine                                                             MW  Megawatts                                                                 LHV  Lower heating value                                                 

It should be particularly noted that in both embodiments of the presentinvention, axial flow through the second combustor is present, givingrise to lower pressure loses which translate into higher over-allefficiency. Moreover, in both embodiments, a longer diffuser is possiblebetween the gas generator and second combustor. In the case of thereheat gas combustor, a two-shaft design leads to higher efficiency. Inthe case of the cavity for gas reheat, combined with steam reheat, thelong diffuser provides for maximum velocity head recovery.

Referring again to FIG. 4, the cylindrical construction of cavity 90 isapparent, leading to equal and even loop, that is, pressure vessel,stress. Coils 190 about the periphery of the inside of cavity 90 defineand in effect form the combustion chamber thereof, and inside coils 190serve as an insulator to shell 176. Moreover, inner coils 142 arearranged about the combustion region 134 to control gas flow and mixing.Plenum 192 serves to stabilize eddy currents before gas enters throughinlet nozzle 154 of power turbine 42. Inlet nozzle 154 is configured toreduce pressure loss, through the shape of outside bell mouth 196 andbullet nose 157. Gib keys 198 permit control and maintenance of axialalignment during operation, and flex legs or trunnions (not shown) allowfor axial expansion. Annular combustion caps 155 when used incombination with burner fuel nozzles 136 provide for even temperaturedistribution. The cylindrical construction of cavity 90, with axial flowof gas therethrough, allows use of a plurality of gas generator andreheat assemblies to permit adaptation to large scale power plants onthe order of 1000 megawatt size.

In view of the lower intermediate pressure generated in reheat cavity90, lower grade liquid fuels or solid fuels in pulverized or powderedform can be used in burner nozzles 136, such as low energy content gasfrom coal, coal derived liquids, shale oil, or other liquid fuels, suchas Bunker "C" type oil, residual oil, crude oil or coal powder ofrelatively poor quality and low energy content per unit volume orweight. Further consequences of the lower intermediate pressure ofreheat cavity 90 are lower parasitic compresion power losses, capabilityof using larger power tubrine blades for better cooling and more ruggedservice due to lower abrasion, erosion and corrosion losses, and alarger over-all power turbine for increased expansion efficiency.

In the case of burning pulverized or powdered coal in burner 136,diffuser 130 will be equipped as shown in FIG. 19 with circumferentialdivider 200 to separate incoming gas into an inner stream 202 and anouter stream 204, with outer stream 204 being given centrifugal motionby conventional streamlined turning vanes 206 to impart a greaterspinning motion to outer stream 204. The combustion products fromcombustion of the coal inside cavity 134 are then directed into theouter spinning stream 208 by the pattern of coils 142 for the solidmatter to be collected on the outer circumference of shell 176 intocollector 210 by centrifugal force and bled off of plenum 192 throughline 212. Combustion gas free of solid matter then enters nozzle 154 asdescribed above.

FIG. 9 shows a graph of cycle efficiency as a function of net workoutput, illustrating the increase of both cycle efficiency and netoutput with increasing firing temperature, but also showing the dropoffof cycle efficiency while net output increases as reheat proceeds fromno reheat to full reheat. In FIG. 9, the subscript designates thepressure ratio, r₆ representing a pressure ratio of 6, and r₂₂representing a pressure ratio of 22, for example. It is important tonote that for full reheat, the optimum cycle ratio for net output isalso the optimum ratio for cycle efficiency. Both rise to a maximum andthen fall off sharply. It is the increase in net work output whilemaintaining a higher cycle efficiency which gives rise to the advantagesof the present invention, utilizing the particular structure taughtherein to attain higher firing temperature of reheated gas.

FIG. 10 discloses the effect of firing temperature and cycle pressureratio, since expansion ratios available for reheat cycles are importantin determining combustor size and pressure drop. Intersection of theoptimum pressure ratio for efficiency with the curves for three firingtemperatures reveals that at 1600° F. (871° C.), the expansion ratioavailable is only about 2.5, while at 2000° F. (1093° C.), the expansionratio increases to 3.75, and at the still higher firing temperature of2400° F. (1316° C.), the ratio increases further to 5.25. Inasmuch asthe power turbine expansion ratio is a measure of the efficiencyover-all of a power generating system, and the gas generator 20described as second generation equipment now available commercially andfiring at 2000° to 2100° F. (1093° to 1149° C.) continuous load, thereheat cycle of the present invention is decidedly more practical thanwith first generation gas generators firing at 1600° F. (871° C.), whichare seen to produce a rather low power turbine expansion ratio.

FIG. 11 shows the effect of another factor to consider in a reheatcycle, namely, the temperature level of gas generator exhaust. FIG. 11shows the dependence of gas generator and power turbine exittemperatures as a function of cycle pressure ratio, showing exit gastemperature at the gas generator in dashed lines, indicating the strongdependence on firing temperature and the appreciable dropoff as thecycle ratio is increased. Solid lines refer to power turbine exittemperatures. FIG. 11 illustrates that power turbine exit temperaturesare not as sensitive to cycle pressure ratios as gas generator exittemperatures. With second generation gas generators, such as thatdescribed above in connection with the present invention, the cyclepressure ratios available range from 18 to 30, and the gas generator hasa 2000° F. (1093° C.) or higher firing temperature, giving exittemperature within practical limits for reheat cycle. It should be notedthat when considering heat recovery, the power turbine exhausttemperature is important. The power turbine exhaust temperature rangesin FIG. 11 are ideal for heat recovery in the combined cycle. It iswell-known that the best range of exit temperatures for heat recoveryboilers is between 1200° and 1400° F. (649° and 760° C.).

When considering a reheat cycle power turbine, the materials ofconstruction for the exhaust plenum 192 must be carefully considered.High temperature materials, however, are within the state of the art, asis cooling of power turbine blading for the reheat firing temperaturesof the present invention.

FIG. 12 discloses another factor to be considered, namely, the amount offuel to be burned in the reheat combustor compared with the firstcombustor. In general, the reheat combustor requires less fuel than thefirst combustor. The proper fuel ratio can be seen from FIG. 12 for therange of cycle pressure ratios of 18 to 30 and for firing temperaturesof 2000° F. to fall between about 1.2 and 1.8.

Firing temperature has a decided effect on combined cycle efficiency forboth simple and reheat gas turbines as shown in FIG. 13. Increasing thefiring temperature from 1600° F. to 2400° F. raises the efficiency froma little over 40% to about 53.5% for the combined simple cycle and fromabout 45% to a little over 56% for the combined reheat cycle. At 2000°F. firing temperature, the reheat cycle has an efficiency pointadvantage of about 3.5%, as illustrated in FIG. 13 by the verticalarrows. This represents a 7.4% improvement over the simple cycleefficiency value, which is the equivalent to an increase of about 200°F. in firing temperature, as indicated by the horizontal arrows in FIG.13. This gain is significant and is worthy of note, considering the highcost of fuel today and projected cost in future years.

It should be noted that any designation of partial reheat in the Figuresrefers to a reheat firing temperature of 1800° F. The reheat firingtemperature of 1800° F. was selected for two basic reasons: first sothat lower cost and readily available power turbine bladingincorporating so called internal convection-blade cooling can be usedinstead of more expensive film-cooled blading, and secondly so that theexhaust gases exiting from the power turbine will be at a preferredtemperature of about 1250° F. for optimum steam generation, that is,minimum stack temperature, and a lower cost of materials andconstruction required for the power turbine exhaust plenum. In thefuture, as the state of the art permits gas turbine firing temperaturesand pressure ratios to be increased to provide higher power turbineexpansion ratios, the reheat firing temperature would be increased tomaintain the optimum 1250° F. temperature range to the heat recoveryboiler. At such time the more expensive film-cooled blades would beemployed. Such future developments would further increase output andimprove cycle efficiency as shown in FIGS. 9 through 15.

An over-all picture of the two combined cycles in terms of cycleefficiency is presented in FIG. 15, showing on the left side the simplecycle gas turbine efficiency for the two optimum conditions ofcompression ratio for output and efficiency. The efficiency rises as theheat is absorbed by the boiler and converted to work, where 100% of theheat obtainable is recovered. At this point, the simple cycle gasturbine is slowly changed to a full reheat cycle by adding increasingamounts of fuel in the second combustor until firing temperatures areequal. The cycle efficiency rises as the efficiency lines run from leftto right.

FIG. 14 illustrates the effect on output for the combined cycle as afunction of firing temperature. Output increases substantially as firingtemperature rises, and moreover, the reheat output has a slightlysharper rise than the simple cycle output. FIG. 14 is useful inevaluating size and potential cost of equipment. For example, with a2000° F. firing temperature, the combined reheat cycle is shown todevelop an increase of about 95 BTU per pound of air flow over thecombined simple cycle, equivalent to approximately 56% more output. Thisfigure is an indicator of the relative physical size of the steam andgas turbines and heat recovery boilers of the two cycles and points topotential cost advantages of the reheat cycle. This statement is made inlight of the relatively small change in the output ratio of the gasturbine to the steam turbine as shown in FIG. 16 as reheat temperaturerises. A further point apparent from FIG. 14 is that the reheat cyclefired at 2000° F. is equivalent to a simple cycle fired at nearly 2600°F. to generate the same output.

Incorporating the superheat and reheat functions in the gas turbinereheat combustor 90 shifts the heat load away from the heat recoveryboiler, economizer 64 and evapoorator 58, and makes it possible toprovide a simpler and less costly heat recovery boiler with less tubesurface area with large temperature differentials between the exhaustgas and the water/steam. A lower cost heat recovery boiler is apparentwith less pressure drop possible.

FIG. 17 illustrates how this is accomplished by showing threeconditions: first for superheating and reheating (condition A), secondfor superheating only (condition B) and third for no superheating orreheating (condition C). This significant heat transfer change and thetemperature differential advantages of the third case withoutsuperheating or reheating for the heat recovery boiler which is part ofthe invention are illustrated by comparison of the characteristics ofthe steam, represented by lines D, E, F and G, as well as the gascharacteristics in each respective condition represented by lines A, Band C, respectively. Note that condition C is not subjected to theboiler approach temperature, which is near the assumed practical limitof 50° F. shown in FIG. 17.

FIG. 18 illustrates particality of the disclosed arrangement by showingthe advantageous temperature differentials for high heat transfer thattakes place in the reheat combustor 90 between the reheat gas and thesteam coils 142 which is predicated on the specific optimum cyclepressure ratio selected of about 29 and the associated 2100° F. gasgenerator firing temperature and the 1800° F. reheat firing temperature.It can be noted that the bypass gas entering at about 1250° F. has amean effective temperature difference with respect to the steam of about400° F. at a typical location shown at arrow H, whereas the combustorcavity gas exiting at about 2450° F. from cavity 134 has a meaneffective temperature difference of about 1200° F. as shown at arrow I.The two vertical arrows H and I illustrate these two areas of heattransfer. It should also be noted that the flame in cavity 134 providesa source of radiant heat that further increases the heat flux availableto minimize the surface of tubes 142 and thus minimize pressure drop.FIG. 18 further shows the bypass gas temperature dropping as the gasheats the steam. The gas temperature is shown to rise again after aboutthe 30% heat transfer point as the gas enters downstream combustion zone134 where the combustion gas mixes with bypass gas.

The considerations outlined in discussion of FIGS. 9 to 18 show thevalue of gas reheat in improved over-all cycle efficiency, therebydemonstrating and substantiating the advantages of the first embodimentof the present invention, providing for gas reheat. The still greateradvantages of the second embodiment of the present invention, providingfor gas reheat and steam reheat, are apparent from the discussion andcomparisons drawn in connection with discussion of the apparatusillustrated in FIGS. 4 to 6. Accordingly, the process of the presentprovides for significantly increased reheat cycle gas turbine output,while the reheat gas turbine efficiency is only slightly degraded overthe simple cycle for equal firing temperatures. Moreover, the combinedcycle incorporating a reheat gas turbine offers significant cycleefficiency improvements for equal firing temperatures, and the outputper unit air flow is significantly greater for the combined cycleutilizing the reheat gas turbine, leading to potential cost savings forsuch a cycle.

Throughout the specification and claims, unless otherwsie specified,cycle efficiencies are expressed in terms of fuel lower heating value(abbreviated LHV), temperatures are given in degrees Fahrenheit,pressures in pounds per square inch absolute, costs in U.S. dollars,power in kilowatts (KW), energy in British Thermal Units (BTU), andparts and proportions in percent by weight. Conversion factors to SIUunits are as follows:

BTU=1.055 kilojoules

BTU per pound=2.326×10³ joules per kilogram

degrees F.=9/5 degrees C.+32

psi=6.8947 kilopascals.

The foregoing is considered as illustrative only of the principles ofthe invention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed, and accordingly, all suitable modifications and equivalentsmay be resorted to, falling within the scope of the invention.Similarly, as the state of the art of gas turbines advances throughimproved higher temperature metallurgy, higher temperature constructionmaterials, higher firing temperatures, higher compressor efficiency,higher turbine efficiency, and higher compressor ratios, the generaltemperature and pressure relationships between the first combustor,second combustor, gas generator and power turbine are considered to moveupward accordingly from the levels presented heretofore and fall withinthe scope of the invention.

What is claimed as new is as follows:
 1. A process for production ofuseful power comprising the following steps:(a) generating in a gasgenerator means a high temperature, high pressure first gas from ambientair and gas generator fuel without the use of intercooling: (b)reheating said first gas in a reheat combustor by injection and ignitionof reheater fuel to form a high temperature, high pressure second gas;(c) driving with said second gas a power turbine for generation ofuseful power in a first generating means and production of an exhaustthird gas; (d) exchanging heat in heat exchange means for said third gaswith liquid water for generation of steam for production of useful powerin a second generating means by a steam turbine, said gas generatormeans comprising a generator turbine drivingly connected by a generatorshaft to a turbine gas compressor, said compressor receiving saidambient air for producing high pressure air and discharge thereof into afirst combustor, said first combustor receiving said gas generator fueland forming a combustion gas for driving said generator turbine, saidgenerator turbine driving said compressor and forming said first gas fordischarge to said reheat combustor, wherein the pressure ratio of saidhigh pressure air to said ambient air is at least about 18 and saidcombustion gas is formed at a temperature of at least about 2000° F.,and wherein said heat exchange means comprises an evaporator and aneconomizer, said evaporator receiving said third gas for transfer ofheat therefrom to produce a fourth gas, said economizer receiving saidfourth gas for extraction of heat therefrom to produce a discharge gas,said economizer receiving liquid water condensate from said steamturbine and receiving heat from said fourth gas for warming said waterand discharging said warmed water to said evaporator, said evaporatorreceiving heat from said third gas for producing input steam from saidwarmed water, said input steam being discharged to said reheat combustorto form superheated steam.
 2. The process of claim 1 wherein saidreheater fuel is a lower grade fuel than said gas generator fuel.
 3. Theprocess of claim 2 wherein said reheater fuel is selected from the groupconsisting of coal gas, coal derived liquid, shale derived liquid,Bunker "C" type oil, residual oil, crude oil, and powdered coal.
 4. Theprocess of claim 1 wherein said first generating means is a firstelectric generator and said second generating means is a second electricgenerator.
 5. The process of claim 4 wherein said first electricgenerator is drivingly connected to a pair of oppositely extendingoppositely rotated driving shafts, each of said driving shafts beingconnected to a power turbine driven by said second gas.
 6. The processofclaim 1 wherein said gas generator fuel is common with said reheaterfuel, both being supplied from a common source.
 7. The process of claim1 wherein said compressor comprises a five-stage low pressure section,followed by a 14-stage high pressure section, and said generator turbinecomprises a two-stage high pressure section followed by a one-stage lowpressure section, the firing temperature of said first combustor toproduce said combustion gas is at least about 2100° F. and the pressureratio of said high pressure air to said ambient air is about
 29. 8. Theprocess of claim 1 wherein said reheat combustor comprises an axial flowchamber having coils therein for receiving said input steam andgenerating said superheated steam, said chamber further including aplurality of fuel nozzles for injection of said reheat fuel into saidchamber.
 9. The process of claim 8 wherein said coils comprise a firstset of coils arranged helically about the outer periphery of said reheatcombustor and a second set of coils arranged helically within saidreheat combustor and spaced from said first set of coils to form anannular space therebetween, said fuel nozzles being placed within saidannular space and adjacent the entrance of said flow chamber to providecontrolled combustion within said annular space, said input steam beingintroduced into said reheat combustor from a steam header, and beingsuperheated by said reheating of said first gas and by said first gascontacting said coils before reheating.
 10. The process of claim 8wherein said first combustor has a rich fuel mixture and said reheatcombustor has a lean fuel mixture to control nitrogen oxide pollutantemissions.
 11. The process of claim 8 wherein said first combustor has alean fuel mixture and said reheat combustor has a rich fuel mixture tocontrol nitrogen oxide pollutant emissions.
 12. The process of claim 8wherein water or steam is injected into said reheat combustor to controlnitrogen oxide pollutant emissions.
 13. The process of claim 1 whereinsaid steam turbine generates an exhaust steam which is passed into saidreheat combustor for heating, producing a reheated steam which isreturned to said steam turbine for assisting in driving thereof.
 14. Theprocess of claim 8 wherein said steam turbine produces exhaust steamwhich is passed into said reheat combustor for reheating in said helicalcoils and returned therefrom to said steam turbine for assisting indriving thereof.
 15. The process of claim 1 wherein said gas generatormeans, said reheat combustor, and said power turbine are combined in alinear axial arrangement and the flow of said first gas from said gasgenerator means through said reheat combustor and to said power turbineis substantially parallel to the linear axis of said arrangement.
 16. Aprocess for production of useful power comprising the followingsteps:(a) generating in a gas generator means a high temperature, highpressure first gas from ambient air and gas generator fuel without theuse of intercooling, said ambient air being compressed within said gasgenerator means before the introduction of gas generator fuel to producehigh pressure air, the pressure ratio of said pressure air to saidambient air being at least about 18, said first gas being produced at atemperature of at least about 2000° F. by igniting said high pressureair and said gas generator fuel; (b) passing said first gas into areheat combustor for reheating by injection and ignition of reheaterfuel thereinto, and for exchange of heat to form superheated steam andfor forming a reheated second gas, said superheated steam being passedto a steam turbine; (c) driving with said second gas a power turbine forgeneration of useful power in a first generating means to produce anexhaust third gas; (d) exchanging heat in heat exchange means from saidthird gas with liquid water for generation of input steam, said inputsteam being discharged into said reheat combustor for producing saidsuperheated steam, said steam turbine being driven by said superheatedsteam for production of useful power in a second generating means,wherein said reheat combustor comprises an axial flow chamber coilstherein for receiving said input steam and generating said superheatedsteam, said chamber further including a plurality of fuel nozzles forinjection of said reheat fuel into said cavity, said coils comprising afirst set of coils arranged helically about the outer periphery of saidreheat combustor and a second set of coils arranged helically withinsaid reheat combustor and spaced from said first set of coils to form anannular space therebetween, said fuel nozzles being positioned withinsaid annular space and adjacent the entrance of said flow chamber.